In a power conversion system such as a variable-speed, constant-frequency (VSCF) power generating system, a brushless, three-phase synchronous generator operates in a generating mode to convert variable-speed motive power supplied by a prime mover into variable-frequency AC power. The variable-frequency AC power is rectified and provided over a DC link to a controllable static inverter. The inverter is operated to produce constant-frequency AC power, which is then supplied over a load bus to one or more loads.
As is known, a generator can also be operated as a motor in a starting mode to convert electrical power supplied by an external AC power source into motive power which may in turn be provided to the prime mover to bring it up to self-sustaining speed. In the case of a brushless, synchronous generator including a permanent magnet generator (PMG), an exciter portion and a main generator portion mounted on a common shaft, it has been known to provide power at a controlled voltage and frequency to the armature windings of the main generator portion and to provide field current to the main generator portion via the exciter portion so that the motive power may be developed. This has been accomplished in the past, for example, using two separate inverters, one to provide power to the main generator portion armature windings and the other to provide power to the exciter portion. Thereafter, operation in the generating mode may commence whereupon DC power is provided to the exciter field winding.
Cook, U.S. Pat. No. 4,786,852, assigned to the assignee of the instant invention, discloses a power conversion system including a starting arrangement in which a brushless generator is operated as a motor to bring an engine up to self-sustaining speed. A rectifier bridge of a VSCF system is modified by adding transistors in parallel with the rectifiers of the bridge and the transistors are operated during a starting mode of operation to convert DC power provided on a DC link by a separate VSCF system or auxiliary power unit into AC power. The AC power is applied to armature windings of the brushless generator to cause a rotor of the generator to be accelerated.
Shilling, et al, U.S. Pat. No. 4,743,777 discloses a starter/generator system including a brushless, synchronous generator. The system is operated in a starting mode to produce motive power from electrical power provided by an external AC power source. An exciter of the generator includes separate DC and three-phase AC field windings disposed in a stator. When operating in a starting mode at the beginning of a starting sequence, the AC power developed by the external AC power source is directly applied to the three-phase AC exciter field windings. The AC power developed by the external AC source is further provided to a variable- voltage, variable-frequency power converter which in turn provides a controllable voltage and frequency to the armature windings of a main generator. The AC power provided to the AC exciter field windings is transferred by transformer action to exciter armature windings disposed on a rotor of the generator. This AC power is rectified by a rotating rectifier and provided to a main field winding of the generator. The interaction of the magnetic fields developed by the main generator field winding and armature winding in turn causes the rotor of the generator to rotate and thereby develop the desired motive power. When the generator is operated in a generating mode, switches are operated to disconnect the AC exciter field windings from the external AC source and to provide DC power to the DC exciter field winding. The power converter is thereafter operated to produce AC output power at a fixed frequency.
Other types of starting/generating systems are disclosed in Glennon, et al., U.S. Pat. Nos. 4,868,406 and 5,068,590, Dhyanchand, et al., U.S. Pat. No. 4,947,100 and Dhyanchand, U.S. Pat. Nos. 4,968,926, 5,013,929, 5,015,941 and 5,055,700, assigned to the assignee of the instant application.
All of the foregoing systems are useful to provide motive power for starting of a prime mover. However, all of these systems utilize brushless wound-field generators having an exciter and a PMG in addition to a main generator. Brushless wound-field generators are relatively heavy and long in the axial direction owing to the need for cascaded electromagnetic stages. Also, the use of a rotating winding and rotating rectifier limits the efficiency, ruggedness and reliability of the generator.
In addition to the foregoing, when a brushless, synchronous wound-field generator is utilized as part of a VSCF system operable in generating and starting modes, it is necessary to provide an exciter and main generator of higher power rating than if the system were used for generating alone. Thus, the size and weight of the generator must be further increased in order to accommodate the starting function.
Recent advances in magnetic materials have permitted the substitution of a PMG for the wound-field generator of a VSCF system. In this case, the PMG may be of the axial type wherein axial flux is developed by permanent magnets carried by a rotor. Control over the output voltage of a PMG may be effected by providing two (or more) relatively movable permanent magnet field structures in proximity to a single armature winding or two (or more) relatively movable, series-connected armature windings in proximity to a single permanent magnet field structure. The relative positions of the field structures or armature windings are varied to control the output voltage of the generator. Such a generator is sometimes referred to as dual permanent magnet generator (DPMG).
The use of a PMG in a VSCF starting/generating system is suggested at column 3, lines 42-45 of the above-noted Dhyanchand '929 patent, although no details are provided as to how this might be accomplished, nor is there any disclosure or suggestion that a DPMG may be used.
Axial-gap DPMG's are disclosed and claimed in Lynch, et al., U.S. patent application Ser. No. 07/693,622, filed Apr. 30, 1991, entitled "Axial Gap Dual Permanent Magnet Generator", now U.S. Pat. No. 5,245,238 and Shah, U.S. patent application Ser. No. 07/931,168, filed Aug. 17, 1992, entitled "Permanent Magnet Generator With Auxiliary Winding", both assigned to the assignee of the instant application and the disclosures of which are hereby incorporated by reference herein.